1. Field of the Invention
This invention relates to the field of purification of low boiling point gases such as nitrogen and oxygen and especially to a process and apparatus for the purification of oxygen in liquid or gas form. The invention is particularly suited to the purification of oxygen produced by standard cyrogenic air separation processes and also to the purification of oxygen obtained from stored cylinders of liquified oxygen.
2. Description of the Prior Art
Standard cryogenic air separation processes involve filtering of feed air to remove particulate matter followed by compression of the air to supply energy for separation. Generally the feed air stream is then cooled and passed through adsorbents to remove contaminants such as carbon dioxide and water vapor. The resulting stream is subjected to cryogenic distillation.
Cryogenic distillation includes feeding the high pressure air into one or more separation columns which are operated at cryogenic temperatures whereby the air components including oxygen, nitrogen, argon, and the rare gases can be separated by distillation. An enriched air product can be obtained through the cryogenic air separation process which ranges from 25% oxygen to about 90% oxygen. It is also possible to produce higher purity oxygen having a purity in the range of 70-99.5% percent oxygen. For example, a stream of oxygen containing 99.5% oxygen contains 0.5% argon and trace amounts of contaminants such as krypton, xenon and various hydrocarbons. In addition, there are trace amounts of nitrogen.
The trace components mentioned above are generally present in parts per million and are not a problem for most applications for the use of oxygen. However, certain industrial processes require extremely high purity levels. For example, the electronics industry presently requires oxygen having a total impurity content of less than 100 ppm. Moreover, the presence of krypton and hydrocarbons are particularly undesirable.
One process which has been suggested for the production of ultra-high purity oxygen is described in U.S. Pat. No. 4,560,397. This process uses a standard double column air separation process and includes a step of withdrawing a vapor stream from the low pressure secondary column at a point above at least one equilibrium stage above the vaporizing oxygen-enriched liquid. This process produces oxygen in gaseous form which for most applications must be subsequently compressed, a process which has the potential to produce undesirable particulates. Also, the process is not suitable for purification of liquified gases stored in cylinders or for oxygen vapor streams withdrawn from standard cryogenic air separation processes which do not fulfill the required high purity standards.
Therefore, it is an object of this invention to provide an improved process for purification of oxygen to produce ultra-high purity oxygen in liquid or gaseous form.
It is a further object of this invention to provide a purification process which is suitable for subsequent purification of both liquid and gaseous oxygen produced by cryogenic air separation processes.
It is a still further object of this invention to provide an improved process for producing ultra-high purity oxygen from oxygen obtained from separate oxygen production processes.
It is a further object of this invention to provide a purification process which is suitable for subsequent purification of both liquid and gaseous nitrogen produced by cryogenic air separation processes.
It is a further object of this invention to provide a purification process which is suitable for purification of nitrogen and other low boiling point gases.
It is a further object of this invention to provide a purification process whereby oxygen obtained from standard storage cylinders can be purified.
It is a further object of this invention to provide a purification process whereby oxygen is purified using nitrogen, oxygen, air or mixtures thereof as the refrigeration medium, which gases may be obtained from air separation or other high purity gas production processes.